This application is related to Japanese patent application No. HEI 11(1999)-118973 filed on Apr. 27, 1999 whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a photodetector and a method of driving the photodetector, and more particularly to a photodetector for a short wavelength (violet color) suitable used for an optical pickup and a method of driving the photodetector.
2. Description of the Related Art
In recent years, there has been a prospect that a violet semiconductor laser (wavelength xcex=approximately 400 nm) using a GaN based material or a violet laser using a SHG (secondary harmonic generation) element can practically be developed, and it has been expected that an optical memory can have a density increased by a reduction in a wavelength of a light source.
However, Nikkei Electronics 1998. 1.26 (No. 708) pp. 127-129 has described that the following problem arises in that a signal is reproduced from a recording mark which is made minute with a reduction in a wavelength of a light source, for example.
Referring to the description, it is hard to reproduce a signal because the sensitivity of a pin photodiode of Si which is a photodetector used in an optical pickup for an optical memory in addition to a decrease in a signal amount (caused by a reduction in a size of the recording mark). The cause includes (1) a decrease in the number of photons caused by a reduction in the wavelength of the light source, (2) a reduction in a quantum efficiency (Although a light absorption coefficient of Si to be the material of the photodetector is increased with a reduction in the wavelength so that a probability of absorption near a surface is increased, a generated carrier is recombined and easily dissipated because there are a large number of surface states near the surface), and the like.
As measures to improve a sensitivity with the short wavelength of the photodetector, an antireflection film is improved corresponding to a wavelength of a violet color or a thickness of an i layer having a pin type structure is regulated. In the pin photodiode having no amplifying function, however, a light receiving sensitivity has limitations as a matter of course. Under such a background, an increase in a density of an optical memory has needed a photodetector and an optical pickup which have an amplifying function that is capable of ensuring a light receiving sensitivity corresponding to a decrease in a signal amount and a reduction in the wavelength of the light source (violet color: wavelength xcex=approximately 400 nm) in addition to the conditions (1) a high speed operation ( less than 10 nsec) and (2) a low operating voltage ( less than 5 V) which are required in respect of a recording and reproducing speed and consumed power.
As the photodetector having the amplifying function, an avalanche photodiode has been well known. For example, Japanese Laid-Open Patent Publication No. Hei 9(1997)-265652 has described an optical pickup using the avalanche photodiode having the amplifying function which serves to cope with a reduction in a sensitivity of a photodetector for a short-wavelength light source having a violet color. However, the avalanche photodiode usually requires an operating voltage of 100 V or more,. Therefore, it is hard to use the avalanche diode as a photodetector for an optical memory.
On the other hand, Japanese Laid-Open Patent Publication No. Hei 10(1998)-261291 has described xe2x80x9can optical switching element using an insulator-metal transition generated by irradiating light on a manganese oxidexe2x80x9d, which is not the photodetector such as the pin photodiode or the avalanche diode. According to this publication, a gold electrode is formed at an interval of 50 xcexcm on a single crystal Pr0.7Ca0.3MnO3, and an insulator-metal transition is caused to greatly change an electric resistance when a YAG (yttrium-aluminum-garnet) laser beam is irradiated while applying a DC bias voltage of several V to the gold electrode. The insulator-metal transition caused by the light irradiation on the Pr0.7Ca0.3MnO3 has also been described in detail n the document xe2x80x9cPhysical Review Letters Vol. 78, No. 22, (1997), pp. 4257-4260xe2x80x9d and xe2x80x9cJournal of the Physical Society of Japan Vol. 66, No. 11 (1997), pp. 3570 to 3576).
However, the use of the optical switching element as described above as a photodetector for an optical memory has a problem in respect of a response speed because (1) a relaxation time (a time required for an insulator-metal transition) is long, for example, several hundreds nsec to several xcexcsec for the generation of the insulator-metal transition. Moreover, (2) since a bias voltage of at least several tens V or more is required for causing the insulator-metal transition by feeble reflected light (several xcexcW) transmitted from a recording bit, there is a problem in respect of an operating voltage. Furthermore, since (3) a metal state is held even if the light irradiation is stopped after the complete transition to the metal state is carried out, it is hard to detect the presence of the recording bit.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a photodetector having a light receiving sensitivity corresponding to a decrease in a signal amount with an increase in a density of an optical memory and a reduction in a wavelength of a light source (violet color: wavelength xcex=approximately 400 nm) by utilizing, as an amplifying function, an insulator-metal transition by light irradiation and capable of carrying out a high-speed and low-voltage operation suitable for a photodetector for the optical memory, a method of driving the photodetector and an optical pickup using the photodetector and the driving method thereof.
The present inventors have paid attention to the relationship of an arrangement between an electrode and a region (referred to as a complex excitation region) in a photoelectric conversion layer where a light irradiation region and a voltage application region are overlapped because of the generation of an insulator-metal transition in the complex excitation region and have found that the insulator-metal transition can be implemented at a high speed and a low voltage by properly setting the relationship of the arrangement.
The present invention provides a photodetector comprising: a photoelectric conversion layer for performing an insulator-metal transition by receiving light; and a pair of electrodes provided opposite to each other in contact with the photoelectric conversion layer, between which a voltage is applied, wherein the pair of electrodes are arranged on the photoelectric conversion layer such that a region of the photoelectric conversion layer which performs the insulator-metal transition and a region to which the voltage is applied through the pair of electrodes are substantially coincident with each other (overlapped almost wholly).